Add caching layer to CI (#277)

Added a caching layer to the CI

This creates a cache on first run (should exist because of this pr now)
The hash is calculated from the Cargo.toml files (recursively if i've
done it correctly)
So whenever a Cargo.toml file changes, it invalidates the cache.

Local crates are _not_ cached, so they get recompiled regardless, but it
already shaves off like half the total time from the action runners.

.github/workflows/ci.yml

@@ -24,6 +24,20 @@ jobs:
       with:
         components: clippy, rustfmt
     - run: cp crates/playground/src/playground.template.rs crates/playground/src/playground.rs
+    - name: Set up cargo cache
+      uses: actions/cache@v3
+      continue-on-error: false
+      with:
+        path: |
+          ~/.cargo/bin/
+          ~/.cargo/registry/index/
+          ~/.cargo/registry/cache/
+          ~/.cargo/git/db/
+          target/
+        key: ${{ runner.os }}-cargo-target-${{ hashFiles('**/Cargo.toml') }}
+        restore-keys: |
+          ${{ runner.os }}-cargo-target-${{ hashFiles('**/Cargo.toml') }}
+          ${{ runner.os }}-cargo-target
     - name: Validate formatting
       run: cargo fmt --all -- --check
     - name: Validate documentation